1. Field of the Invention
This invention relates to an improved process for the production of maleic anhydride by the catalytic oxidation of n-butane. More particularly, the invention pertains to a process for the production of maleic anhydride by the catalytic oxidation of n-butane having an improved yield and selectivity by the addition of a substantially pure carbon monoxide gas stream in the reactant feed stream.
2. Description of the Related Art
It is well known in the art that maleic anhydride may be manufactured by the vapor phase oxidation of n-butane as it flows through a fixed bed reactor containing a vanadium phosphorus oxide (VPO) catalyst. N-butane in admixture with air is brought into contact with a VPO catalyst under conditions such that the n-butane is oxidized to maleic anhydride. The effluent from the reactor may be cooled to partially condense the product maleic anhydride from the effluent gases. The gaseous product, with or without partial maleic anhydride removal, is scrubbed using a solvent, usually water or an organic solvent, to recover the maleic anhydride. The remaining gases, containing unconverted n-butane, are commonly incinerated in an effluent gas incinerator prior to venting to the atmosphere. Useful processes, as well as VPO catalysts for the production of maleic anhydride from n-butane are known from U.S. Pat. Nos. 6,194,587; 5,360,916 and 6,858,561, which are incorporated herein by reference. U.S. Pat. No. 5,360,916 discloses a two-stage process for maleic anhydride production wherein n-butane is oxidized in a first reaction zone and the effluent from this zone is passed together with supplemental n-butane to a second reactor in series in order to complete production of the maleic anhydride. In U.S. Pat. No. 5,360,916, butane is oxidized with molecular oxygen in a dilute state by bringing a mixture of vaporized butane and air having a controlled butane content into the presence of a vanadium phosphorus oxide catalyst in a first oxidation zone, cooling the gaseous effluent, introducing butane into the cooled gaseous effluent from the first oxidation zone, introducing the cooled butane-enriched stream into the second oxidation zone, and bringing the thus butane-enriched mixture into contact with a vanadium-phosphorus-oxygen catalyst disposed in the second zone. U.S. Pat. No. 6,858,561 teaches a process for preparing a catalyst for maleic anhydride production.
There is a need in the art to improve the selectivity and yield of maleic anhydride production processes. It has now been unexpectedly found that the selectivity and yield of the maleic anhydride production process may be increased by adding a stream of carbon monoxide gas to the inflowing reactant gas streams. This is counter-intuitive. One skilled in the art would presume that the addition of carbon monoxide would tend to reduce selectivity and yield of the maleic anhydride production process.
After recovering the produced maleic anhydride, preferably at least part of the residual gas from the reaction product is then recycled back to the input streams. In the conventional once-through butane oxidation to maleic anhydride process about 80% of the butane is converted to maleic anhydride and by-products. The unconverted butane needs to be disposed of, and is usually burned off as a fuel gas. To improve the yield, i.e., the fraction of butane fed converted to maleic anhydride, the butane can be recycled. In the conventional once-through process, the feed gas is free of carbon monoxide and carbon dioxide reaction by-products, except for a small amount of carbon dioxide which might be contained in the air feed. However, as a consequence of butane recycle the concentration of these by-products will build-up in the recycle gas. In addition, for the recycle process the oxygen concentration can be varied at will, while the once-through process is generally carried out with air and is restricted to 20.9% oxygen. U.S. Pat. No. 5,688,970 teaches a process for the production of maleic anhydride by recycling at least part of the residual gas from the reaction product back to the input streams. However, prior to doing so, the process of U.S. Pat. No. 5,688,970 requires a scrubbing of the residual gases so that only recycled and scrubbed butane re-enters the reaction.